Which are safer: piston twins or singles?

[QDMQDMQDM]

I don't have the stats, but would be interested to hear people's opinions.

My logic works like this:

Most piston twins are marginal with one engine out, requiring excellent technique to survive, particularly at critical phases of flight. Since they have two engines, they are therefore twice as likely to have an engine failure and are therefore less safe. This is especially true when flying on ferry flights, such as transatlantic, at or above max gross, because some of them can't keep flying on one engine at all at such weights. They're therefore twice as likely to have to ditch due to engine failure. Alternatively, if they can keep flying, the drag goes up, airspeed goes down and they end up without enough fuel to make land.

And the above does not even take into account the increased complexity of these aircraft and therefore the drain on pilot attention etc. etc.

Or, to coin a phrase: "When the first engine fails in a twin-engined aircraft, the second engine takes you to where the crash is going to be."

QDM

[tacpot]

My opinion would be the same. With a single engine, you don't don't have any options, so the situation is easier to manage in my mind. Also if you know you are relying on that single engine, I think you take additional precautions compared to when you have the 'safety' of a second engine.

Twice the engines does indeed mean twice the chance of failure, but the theory is that the chances of BOTH engines failing are much less than the chance of a single engined aircraft's engine failing. But this is only helpful if the twin has useful single engine performance. As you say, too many do not.

Burt Rutan's Boomerang shows that twins can be built with very much more carefree handling that 'conventional' twins, and of course Cessna's 337 should also have the same carefree handling with an engine out (But I'm not sure what the 337 is really like - there may be some funny situations or rules to be followed depending on whether it is the front or the back engine that has failed!)

[bluskis]

When an engine fails on a single you have a forced engine out landing to perform, perhaps from above cloud, perhaps with poor visability below.

When an engine stops on a twin??

There is no valid arguement.

If you can't cope with an engine out on a twin you assuradly can't cope with an engine out on a single.

[Chimbu chuckles]

Mishandling causes the fatallities not the situation itself...more often than not.

I have had 2 engine failures(+an inflight deliberate shutdown) in 'light piston twins' from mechanical/system failures of one kind or another. One in cruise and one just as the gear tucked up after takeoff, so I feel qualified to comment in both areas.

In the cruise case there was ample time to fiddle in an effort to gain some relief before it finally 'just stopped'...it was caused by multiple magneto failure. I was able to fly to an airfield and land 'normally'. I was in an Islander, 300hp variant.

The takeoff case was caused by a small piece of rubber fuel bladder blocking the uptake, momentarily and repeatedly, causing alternate full power and 'maximum quiet':eek:.

At 50' with high trees coming up quick you decide which one is failing when the aircraft is yawing left and right!

I didn't have a 'dead foot'. Within seconds I had ensured Mix, pitch, pwr up...wheels and flaps up...and it yawed left first so I reduced power to that engine which improved the situation no end so I feathered left. This was in a C402A with only two pax and little baggage and it still took about 5nm to reach 500'...by then past the trees and over water.

What made the difference was training and experience...I was employed by a large 3rd level airline, Talair, which at the time was the largest airline of it's type in the Southern Hemisphere. Every 6 months we had to complete Base Checks and IR/Licence renewals to a high standard...or else...and we flew lots of sectors 6 days a week.

All our 402s and Islanders were OLD but generally well maintained...having said that we suffered engine (and a prop failure once) failures from time to time...the ONLY time fatalities occured was when a combination of DA and terrain made them impossible(or at best HIGHLY UNLIKELY) to survive type situations.

I've had several engine failure/forced landings in singles too. One after takeoff and I turned around and landed with the engine producing negligable power (C182)...one caused by running a tank dry at too low an altitude to effect restart, caused by confusion between myself and my trainee after I asked him to change selection but didn't check he had...MY fault!!

On that occassion I landed on a road downhill, downwind, around a corner in traffic and hit a power pole support guy wire about 50' above the road...which pitched us nose down at the last second before ripping out of the ground and doing some moderate damage as the bitter end with a (now) bent turn buckle whipped around the aircraft. We landed on our wheels and rolled to a halt.

This was in a C185. If I had gone for the tank selector straight away there is a strong likelyhood that the engine would have restarted and only a quick heartbeat result...HOWEVER...this aircrafts tank selector was out of sight between the seats and I was used to my OWN 185 which only had a firewall shutoff...no tank selector...so when ****s were trumps I reverted to what I knew best and the selector didn't occurr to me until I was on short finals to the road...and busy with missing power lines and cars, FLYING THE AEROPLANE...I elected to leave it when I remembered it because I was too busy flying the aeroplane..we hit the support cable a second or two later...if I'd been 'head down' fiddling with the inconveniently placed selector we may have died!

I've also landed a C206 blind after oil covered the windsceen in an engine failure...into a HUGE paddock using only heading and VSI info...the aircraft was badly damaged but I walked away without a scratch...because I flew the aeroplane.

So in which aircraft type was I safer?

Yes you are twice as likely to suffer an engine failure in a piston twin...but the chances of that failure happening at the worst moment...airborne and unable to stop/reland on remaining runway...are low indeed so a SE cruise/driftdown to an airport is entirely feesible.

Training and practice make the difference in the takeoff case...perhaps PPL/CPL holders should not be allowed to fly light piston twins unless they are prepared to spend the money on 'airline' style recurrency training or work for such a company.

I can gaurantee the bleak statistics for this category of aircraft would be dramatically better if this was the case...but an impossible situation to conceive of in a free democratic society..so we put up with the consequences which flow from that.

In the single engine case how often are you completely out of options for places where you can land survivably? RARE INDEED, and in most cases if you FLY THE AEROPLANE you won't even do significant damage to the aeroplane.

If we remove fuel mismanagement from the stats on SE forced landings it becomes a truly rare event...think about THAT!!!

Now think about how many SE forced landing events, by PPLs or low time CPLs end up tragically for NO REASON!!

The recent crash of a Cherokee 6 at Hamilton Is is a classic example....nothing short of the WING FALLING OFF is reason for a situation being unsurvivable for all or most of the occupants.

IT IS THAT SIMPLE!!!!

Talk of situations where operation is impossible, assymetrically, at significantly in excess of MTOW is stupid....no aeroplane will perform in those sort of 'ferry overgross' situations...no-one pretends they will, least of all the manufacturers...they were not designed too so they won't..simple!

If you're ferrying a light piston twin across vast oceans you're in a single engined aircraft until such time as you've burned enough fuel to be below normal weight limits...professionals who do this know it!

So which aeroplane type is safer?

They both are adequately safe when flown properly...I own a single engined aircraft and would not think of changing it for a twin simply because of engine failure considerations.

I could easily sell my Bonanza and buy a similar vintage Baron at no extra cost to purchase, probably even come out ahead...initially.

BUT I would double my engine maintenance and fuel bills for really no huge gain in 'safety'.

If I lived in a country where the weather was really bad a lot or was doing long overwater flights a lot I would buy the Baron or perhaps a C310 but here in Australia where the weather is severe clear 90+% of the time and we have plenty of 'wide open spaces', why?...when a little 'soft IFR' is required in my Bonanza a good hard look at the route and expected cloudbase enroute/freezing level is usually all that is required to give some extra margin for the rare case of MECHANICAL FAILURE.

I don't fly IFR/night SE over tiger country or in severe weather...I wouldn't in a Baron either...it's no more likely to cope with severe turbulence/icing than the Bonanza.

I have flown over SEVERE tiger country(PNG) in piston twins (BE65/C402/404/Aerostar) IFR and in IMC for many thousands of hours...now I don't have to I chose not to...that's what Transport Cat aircraft are for...the ONLY AIRCRAFT with GAURANTEED PERFORMANCE.

Chuck.

[Philip Whiteman]

Although it is comparing apples with pears, the one statistic I do have to hand—published in the last issue of General Aviation—is that TURBINE singles have approximately half the fatal accident rate of piston twins (five per million flight hours vs. 12.5).

Given the reliability of turbine engines, I think you could argue this says something positive about the safety of piston twins!

[nonradio]

Having 2 engines doesn't mean double the risk of failure - that's not the way probability works!
The fatal accident rate for twins after engine failure is higher than that for singles, due to the twin pilot's mindset that the other engine will allow him to fly away under all circumstances.....
Clearly having 2 engines under the right conditions gives you a second chance to keep going.

[StrateandLevel]

The statistics probably show that per hour flown, light twins have a higher accident rate than comparable singles but that does not make them any less safe. The cause is not necessarily the aircraft!

When operating at low altitudes, 2000 ft and below, an engine failure in a single means you have to land somewhere very soon, in a twin its no problem.

The take off case in a light twin is the critical phase of flight but the probability of a failure at that point is very low. If you have planned your departure you should know what isn't possible and plan to land straight ahead if it goes wrong, you could be in exactly the same situation with a single.

Provided you handle a twin correctly, it is as safe as any single and when one quits, I'd say a lot safer as you can keep flying. I'll take the twin any day. Twice the chance of a failure, but half the chance of it being a probem.

[rustle]

There are a lot of other "safety features" not mentioned so far in a twin - eg:

Dual suction/vacuum pumps
Dual Alternators
Multiple fuel "routes" (crossfeed etc.)

You do not have to have an engine failure to have a problem.

=====================

Heavy IMC, cold, above freezing level.

Vacuum pump goes in a single. Uh-oh.... Hope my partial panel skills are current.

Twin: smile - you have another one still sucking merrily (!!)

=====================

Same wx, this time the (only) alternator goes in your single.

Yikes - better land PDQ before my de-ice stops, I have no idea where I am, and I cannot talk to anybody on the ground.

Twin: no problem - you have two alternators and you know the load one-out will allow.

=====================

Apples and oranges - simple comparisons reveal only half a story.

[Flyin'Dutch']

Horses for courses.

I have recently been thinking a lot about this subject. I have a twin rating and am working on my IR.

The main reason for going the IR route (already having an IMC rating) is to make touring to the near continent less weather dependable.

In that scenario only a twin will give the added ability.

Even allowing for the lack of professional retraining, higher allowances for Wx etc I think that if you fly a SE machine you will have too much lack of redundancy of systems to make IFR operations a safe option.

As mentioned here before the 2nd engine does not make an engine failure twice as likely.

There is an interesting review on fatal accidents in Senecas on the pplir website. www.pplir.org IIRC.

To my simple mind the difference between a single and a twin is the following.

Single easier operation in critical flight phase but longer exposure to failure. Twin more difficult operation in critical flight phase but failure exposure a lot shorter.

Either scenario can be enhanced (or aggravated) by some careful pre mission planning.

I.e. Dont fly the twin near gross; make sure you are current; choose sensible Wx minimums etc.

Those who have read the AAIB report about the Titan crash up north will have had the same chilling shivers along their spine as I did when you realise that even highly experienced and trained crew can be involved in an engine out accident like that one.

MHO of course

Frank

[foxmoth]

which is safer depends on many factors, the most important of which is probably pilot experience and currency. Aircraft type, loading, wx conditions are other factors which will influence the result.

[bookworm]

Excellent points from Frank and Rustle, but I think the equipment redundancy thing is being oversimplified.

I've never had an alternator failure, but I have had a total electrical failure. A spare alternator doesn't help if the bus breaks.

Similarly, probably the majority of AI failures I've come across are not due to failure of the vacuum pump, but rather the instrument itself.

As for fuel, I'm certain that the increased complexity of the plumbing in twins has been responsible for at least as many accidents as it has saved -- albeit pilot-induced accidents!

I have a share in both a single and a twin, and fly both happily IFR. That said, there are circumstances when I'm more comfortable in one than the other!

[Cyclic Hotline]

Some statistical research information here. (http://www.pilatus.tv/engine.htm)

Makes some interesting reading.

There are a couple of other very good threads currently running elsewhere on PPRuNe regarding single and twin engine safety that are worth reading.

Rotorheads view (http://www.pprune.org/forums/showthread.php?s=&threadid=70070)


Dunnunder view (http://www.pprune.org/forums/showthread.php?s=&threadid=70118)

As engine failure rates are calculated solely on the ratio of failures per 100,000 flight hours, the statistical probability of a single engine failure on a twin are in fact doubled. In the wrong machine at the wrong time, your prospects are very limited.

Chimbu, you're experiences there make very interesting reading.

[Evo]

The PPL/IR article Frank mentions is here:

http://www.pplir.org/journals/old/NETWORK%20No.%2027.pdf

page 14. You need Acrobat reader to view it.

[411A]

Other factors to consider here as well, with the proviso that, for discussion, a particular pilot is proficient in both types...

If generally flat terrain (Florida, mid-western USA for examples) the single is quite OK, even IFR/night, provided however that the ceiling is reasonable. If the ceiling is below 1000 agl, little opportunity for a forced landing. In this case, the twin is a better bet. Overwater/high mountains etc, most of course would prefer the twin.
Piston engines today, properly maintained, are very reliable, and most accidents in singles are not the result of engine failures.

Turbine singles are very popular now for those that can afford them and although engine reliability is superb, sometimes engine accessories can do you in. Take for example PT-6 powered singles. If the propellor governor pilot shaft breaks, the prop will feather immediately, leaving you with a very expensive glider...with the engine still running. Have had this happen twice on PT-6 powered twins...sure was glad for that second engine.

[Ray Ban]

There is some good gen here. Very informative and interesting posts from Chimbu Chuckles, Frank and rustle. Keep 'em coming!